This invention relates to the reduction of mixtures of iron ore and/or waste oxides and carbonaceous reductants on the hearth of a furnace.
For environmental and economical considerations, it is desirable when reducing iron ore and/or waste oxide by carbonaceous reductants on the hearth of a furnace to use iron ore concentrates directly without high temperature agglomeration and to use coal directly without coking.
The kinetic behaviour of a mixture of fine particles of iron ore concentrate and pulverized coal is very different from that of conventional iron ore agglomerates and coke lumps. In conventional ironmaking processes (e.g. blast furnace, Midrex, Hyl, SL/RN, etc.), the overall reaction rate increases with the increase of temperature but with diminishing effectiveness at higher temperatures. This is due to the fact that, at high temperatures, gaseous diffusion through gaseous boundary layers and metallic shells of agglomerates usually limits the reaction rate. Mass transfer in the gaseous phase is somewhat dependent on process temperature and almost independent of the total pressure in the system. Thus, there is little opportunity for operators to manipulate the system for higher reaction rate.
With a mixture of iron ore concentrate and/or waste oxides and pulverized coal, the situation is different. The interfacial area of reacting solids (ore and carbon) are very large, hence potentially resulting in a fast reaction rate. The distance between ore and coal particles is of the order of tens of microns, i.e., close to the mean free path of gases at higher temperatures. In view of the fact that the reactants are next to each other, resistance to overall reaction due to mass transfer of reactant to reaction sites is not significant. The accumulation of gaseous products (H2, CO, CO2 and H2O) at reaction sites will cause the total pressure to rise. When the overall reaction rate is high, i.e. with higher gas pressure in the interior of the mixture, a viscous flow down the pressure gradient of the system will develop. In this case, gaseous diffusion of reaction products will not play a role in limiting the reaction rate.
However, although such an ore/coal mixture has large interfacial area and little resistance to mass transfer, the actual reaction rate is limited by interfacial chemical reactions which are sensitive to actual temperature at reaction sites. The interfacial temperature is a compromise between heat fluxes in the system and is adjusted to the momentary rate of heat transfer to the location and the rate of heat consumption by endothermic reactions in situ. At higher temperatures, the overall reaction rate of ore/coal mixture is likely to be limited by heat transfer. This has been confirmed by the Ph.D. thesis of S. Sun, accepted by McMaster University, of Hamilton, Ontario, Canada in August, 1997.
With an ore/coal mixture of fine particles, the major elementary steps in the overall reaction are:
(a) heat transfer to the surface of sponge, then to the reaction sites in the interior;
(b) carbonization of coal;
(c) reduction of iron oxide by CO and H2;
(d) gasification of carbon by CO2 and H2O to produce CO and H2;
(e) flow of gas from the interior to the surface of the sponge.
In order to use coal efficiently, gases from step (e) should be collected and burned to generate heat to sustain endothermic reactions in steps (b), (c) and (d).
There is a major problem in the practice of step (a), namely the delivery of heat from an oxidizing flame to the surface of the sponge iron without re-oxidizing it back to iron oxide. It is known to resolve this problem by placing a physical barrier (as well as the medium for heat conduction) between the oxidizing flame and the sponge. Another approach, which is the practice of the INMETCO commercial operation in their rotary hearth furnace, is to prevent the flame from becoming dangerously oxidizing by introducing additional fuel to the flame. Heat is transferred directly to the surface of pellets of waste oxides and coal/coke mixture from a flame with a CO/CO2 ratio of at least 2. However, these solutions have not been particularly satisfactory in practice with respect to energy efficiency.
There are other limitations in current practice. The height of pellet bed is usually about 20 to 25 mm, and in INMETCO practice the bed usually has no more than three layers of pellets. This limits the productivity of a rotary hearth furnace. In a higher bed with reductants of low volatile matter contents, the pellets below 20-25 mm from the top of the bed have difficulty in reaching a high degree of reduction even after a very long time. Metallurgical coke and low volatile coal are preferred as reductants. The temperature of the system, usually expressed as flame or furnace temperature, is usually about 1350xc2x0 C. because higher temperature may cause slag formation and re-oxidation of sponge iron.
It is therefore an object of the present invention to provide an improved process for reducing iron ore and/or waste oxides in mixtures containing carbonaceous reductants.
It is understood that the term xe2x80x9core/coal mixturexe2x80x9d stands for mixtures of ore and/or waste oxides and carbonaceous reductants. The mixture may be in the form of pellets or briquettes.
The present invention is based on the discovery that the reduction of iron ore in an ore/coal mixtures can be significantly improved by changing several parameters simultaneously, as follows:
Accordingly, the present invention provides a process of reducing iron ore in an ore/coal mixture on the hearth of a furnace including providing a bed of iron ore/coal mixture on the hearth of a furnace, the bed having a height of at least about 40 mm and at least four layers of agglomerates, and the coal containing volatile matter with a weight of at least about 10% of the weight of the coal, and heating the bed of iron ore/coal mixture with a radiant heat source having a temperature of at least about 1450xc2x0 C. to cause the top of the bed to reach a temperature in the range of from 1350 to 1530xc2x0 C. to reduce iron oxides in the iron ore and/or waste oxides to metallic iron.
The thermal and chemical reactions in pellets/briquettes bed are independent of the shape of furnace and the movement of the hearth. The hearth in a furnace may be stationary as in our laboratory, or in a linear, rotational, or back-and-forth movement.
The atomic ratio of total carbon in reductants to combined oxygen in iron oxides in the mixture is preferably in the range of from about 0.7:1 to about 1.1:1, and more preferably in the range of from about 0.9:1 to about 1.0:1. The furnace temperature is preferably in the range of from about 1450 to about 1650xc2x0 C., and the bed height is preferably higher than 60 mm (see FIG. 4).
The mixture may be provided as iron ore/coal agglomerates, with the bed having at least four layers thereof. The agglomerates may be iron ore/coal pellets, and the pellets may be in the size range of from about 10 to about 20 mm. Alternatively, the agglomerates may be briquettes of similar size.
The carbonaceous reductants may contain volatile matter with a weight on the average in the range of from about 10 to about 45% of the weight of the coal.